1. Field of the Invention
The present invention relates to apparatus and methods for reacting a catalyst with a fluid, such as for the placement and operation of an ozone decomposing catalyst in a fluid stream.
2. Description of Related Art
Catalysts are used in a variety of applications to alter chemical compositions or remove unwanted contaminants. Such applications include both passive and active catalyst installations. In passive catalyst installations, a catalyst is installed in a fixed position (e.g., near a contaminant generation point in an apparatus) and it reacts with chemicals in a fluid that happen to flow past it. In active catalyst installations, a catalyst is installed in a system whereby a stream of fluid is forced past the catalyst (e.g., in a reactor process vessel, a liquid conduit or an air duct) to encourage the chemical reaction to occur.
One of the primary concerns in the use of catalysts is how to easily and securely install the catalyst while assuring that maximum surface area is available for reaction between the catalyst and the fluid to be treated. To this end, powders or other particle forms of catalyst can provide maximum contact area, but only if the particles can be arranged to permit the fluid to flow freely across their surfaces. This condition may make it difficult to install powdered catalysts in many applications since space is often limited and catalysts generally cannot be freely dispersed in a fluid stream. Further complicating the use of particulate catalysts is that they are not easily installed or retained in place. The attachment of catalysts to a substrate may make them much easier to handle, but inevitably reduces the exposed surface area of the catalyst.
An area of catalyst treatment that has received particular interest is in the use of catalysts to remove environmental contaminants from air and liquid fluid streams. For example, ozone decomposing catalysts are beginning to be recognized as superior to use of merely ozone removing chemicals (e.g., activated carbon) for many uses, such as in air purification systems (e.g., aerospace vehicle air supply), water disinfection applications, industrial corona treatment equipment, and photocopy machines and similar devices. Carbon and similar materials are not fully acceptable in these applications since these materials are consumed in the filtration process and its is often difficult to ascertain when these materials have reached the end of their useful life. By contrast, catalysts have much longer operating lives, decrease the amount of waste generated by the purification system, and tend to be more successful at removing contaminants over a longer period of time. While significant improvements can be realized with the use of catalysts in these applications, regretfully, the use of catalysts in these applications suffers from the installation problems outlined above.
One approach in supplying a catalyst is to bind the catalyst to the surface of a substrate. One example of this method is shown in U.S. Pat. No. 5,187,137 issued to Terui et al., where a thin film coating of manganese dioxide, lead, or lead oxide catalyst is bound to a support (e.g., an inorganic or metal or metal oxide material), preferably using a binder such as alumina sol. Another approach is disclosed in UK Patent 2,258,622 issued to Nichlas Corporation. In this instance, a fine powder of manganese dioxide is coated on a porous carrier paper made from inorganic fibers. Again, a binder such as alumina sol or polyvinyl alcohol is used to hold the catalyst in place. The patent warns that the amount of binder used should be kept to a minimum to avoid coating the catalyst and reducing its catalytic activity. Similarly, in U.S. Pat. No. 5,142,323 issued to Yoshida, a coating of catalyst, foaming agent, and binder is applied to a tape that can be installed in critical area within an electrostatic charge generating machine. All of these approaches are believed to have a number of deficiencies.
First, as has been acknowledged, the use of binders necessarily coats the catalytic particles in order to anchor them and reduces their effectiveness. While too much binder can severely reduce the function of the catalyst, too little binder may limit the amount of catalyst that can be applied in a given area or risks contamination of the fluid stream with catalytic particles. Of course, performance continues to degrade over the life of the filter as shedding continues.
Second, catalysts applied on the surface of a support are not well protected from attack or damage. For instance, surface particles can be fouled by contaminants carried in the fluid stream, such as dust, water, or water vapor. Additionally, the placement of catalytic particles on the surface of a support may leave them susceptible to contamination, damage, or loss where mechanical contact with the particles occurs or where washing or other treatment is necessary.
Japanese Laid-Open Patent Application 4-235718 to Japan Vilene Company, Ltd. dispenses with the use of a binder by embedding an ozone decomposing catalyst within a fibrillated polytetrafluoroethylene (PTFE) resin. By mixing the catalyst within the PTFE resin and then subjecting the mixture to a cutting or shearing force (i.e., performing blending, rolling, or extrusion), the catalyst can be securely bound within a fibrillated PTFE structure without considerable coating of PTFE polymer and without the introduction of a binder material. The patent goes on to teach that voids should be formed in the final product through: blending, rolling, or extruding; introducing a foaming or pore forming agent; or embossing or perforating the support material. The use of pore forming agents is identified as being particularly beneficial, such as using alcohol, polyvinyl pyrrolidone, dimethyl phthalate, diethyl phthalate, ethylene glycol, or naphtha. Suggested foaming agents include inorganic foaming agents (e.g., sodium hydrogen carbonate, ammonium carbonate), azo compounds, and sulfonyl compounds. The patent reports good ozone removal properties by passing a stream of air perpendicularly through the PTFE material. This is attributed to a high probably of contact between the air to be filtered and the catalytic particles within the support material as the air is passed through the PTFE material.
Although the material disclosed in Japanese Laid Open Patent Application 4-235718 appears to address some of the previous problems in binding a catalyst to a substrate, this material continues to have a number of serious limitations. First, the Japanese application does not address how to produce an improved support material that is useful where only cross-flow surface filtration is desired. For many applications flow-through filtration demands excessive pressure drop that cannot be readily accommodated. For instance, cooling fans in electrostatic copiers are intentionally limited in size to reduce noise and electric consumption. A flow-through filter such as that disclosed in the Japanese Application is believed to be impracticable for these uses since too much air pressure will be required to maintain flow through the filter. Pressure drop may be even further increased over time when particles become embedded within the filter during operation, reducing filter effectiveness and requiring even greater air pressure to achieve proper air flow.
Furthermore, the material disclosed in Japanese Laid-Open Patent Application 4-235718 has limited void size control and distribution. That reference teaches that voids are produced by using a pore forming agent which generates very inconsistent void sizes that are difficult to control. The size and distribution of the void spaces within the catalyst material is critical to allowing the gas to reach the catalyst within the depth of the material. Since the efficiency of a filter can be significantly limited by the availability of the catalyst to the contaminated air, it is crucial that void volume is carefully and predictably controlled. the processes taught by the Japanese reference simply does not supply the predictability and control necessary to produce a fully acceptable filter device, particularly one to be employed in cross-flow filtration.
For these applications a cross-flow filter, where air flow occurs only across the exterior surface of the substrate, would be preferred. Unfortunately, the fibrillated PTFE material produced in the methods taught in the Japanese Application is considered to be of too limited porosity to allow effective air exchange between the exterior surface of the substrate and embedded particles in a cross-flow installation.
Another problem with the support material disclosed in Japanese Laid Open Patent Application 4-235618 is that the PTFE material described in the Patent Application is believed to be of limited strength, making it susceptible to leakage or even catastrophic failure. One possible solution to this problem might be to provide some form of support material to reinforce the PTFE material. This solution, however, may only increase the pressure drop across the filter.
Finally, the methods taught in the Japanese Application for producing the support material are considered quite dangerous due to the combustible combination of certain manufacturing chemicals and catalysts such as manganese dioxide. It has been determined that the combination of manganese dioxide with certain organics (e.g., isopropyl alcohol, naphtha, or ethylene glycol) results in the oxidation of the organics. This is particularly likely to occur at elevated temperatures. As a result, the processing of PTFE, manganese dioxide, and an organic foaming or pore forming agent must be carefully controlled to avoid risk of severe combustion. This condition is actually worsened when the combustible materials are removed by volatilization at elevated temperatures, as the Japanese Application recommends, or when the volatilization is carried out in a closed container and/or at elevated pressures. Lastly, even without such processing constraints, it is quite difficult to remove the processing chemicals completely following production. This can leave contaminates that may reduce the effectiveness of the catalyst in operation.
Accordingly, it is a primary purpose of the present invention to provide a substrate for containing a catalyst that securely contains catalyst without the use of a binder or other adhesive material that might reduce the catalyst's effectiveness.
It is another purpose of the present invention to provide a substrate for containing a catalyst that is both highly porous and contains controlled and evenly distributed pores throughout the material, allowing effective exchange of air between the exterior of the substrate and catalytic particles contained therein, even in cross-flow filtration installations.
It is still another purpose of the present invention to provide an improved cross-flow filter for the catalytic treatment of a fluid stream.
It is yet another purpose of the present invention to provide a method of producing an improved substrate for containing catalytic particles that can be safely practiced without risk of combustion.
It is a further purpose of the present invention to provide a method of producing an improved substrate for containing catalytic particles that avoids the introduction of performance diminishing contaminants during processing.
It is another purpose of the present invention to provide a catalyst containing material that is strong and flexible.
It is yet another purpose of the present invention to provide a catalytic filter that can be electrically grounded, charged or heated.
These and other purposes of the present invention will become evident from review of the following specification.